1. Field of the Invention
The present invention relates to an active matrix liquid crystal display (AMLCD) including an active panel with thin film transistors (TFTs) and pixel electrodes arranged in a matrix pattern, and a method of manufacturing the same. More particularly, the present invention relates to a method of manufacturing a color filter panel having a high aperture ratio for an AMLCD, and the color filter panel.
2. Description of the Background Art
Among display devices for showing visual images on a screen, thin film type flat panel display devices are highly favored because of their light weight and easy adaptability. Especially, research activities have actively focused on the development of liquid crystal display (LCD) devices because of their high resolution and fast response time suitable for display of motion picture images.
A liquid crystal display device operates by using polarization and optical anisotrophy of a liquid crystal. By controlling the orientation of liquid crystal molecules having rod shapes using a polarization technique, transmission and interception of a light through the liquid crystal layer are achieved due to the anisotrophy of the liquid crystal. This principle is applied to conventional LCD devices.
AMLCDs having TFTs arranged in a matrix pattern and pixel electrodes connected to the TFTs provide high quality images and are now widely used. The structure of a conventional AMLCD will now be described, referring to FIG. 1.
A conventional LCD 10 includes top and bottom panels joined together with a liquid crystal injected therebetween. The top panel contains elements for reproducing color, and is called a "color filter panel." The color filter panel includes a sequential arrangement of red (3R), green (3G) and blue (3B) color filters formed on a transparent substrate 1a in a matrix pattern. Between these color filters 3R, 3G and 3B, a black matrix 5 is formed in a lattice pattern. The black matrix 5 prevents mixing of the colors at the boundary areas. On the color filters 3R, 3G and 3B, a common electrode 7 is formed. The common electrode 7 is one of the two electrodes used to generate an electric field across the liquid crystal layer when operating the LCD device.
The other panel of the conventional LCD includes switching elements and bus lines for generating the electric field to activate the liquid crystal and is called an "active panel." The active panel of the conventional LCD includes pixel electrodes 11 arranged in a matrix pattern and formed on a transparent substrate 1b. Along the horizontal direction of the pixel electrodes 11, signal bus lines (gate lines) 13 extend with a predetermined distance therebetween. Along the vertical direction of the pixel electrodes 11, data bus lines (source lines) 15 extend with a predetermined distance therebetween. At one corner of each pixel electrode 11, a TFT 17 for driving the corresponding pixel electrode 11 is formed. Each TFT 17 includes a gate electrode 23, a source electrode 25, and a drain electrode 27. The gate electrode 23 of the TFT 17 is connected with the corresponding gate line 13. The source electrode 25 of the TFT 17 is connected with the corresponding source line 15. The drain electrode 27 of the TFT is connected to the corresponding pixel electrode 11. Additionally, pads as the terminals of the bus lines are formed at the end portion of each bus line.
The color filter panel and the active panel are bonded together to face each other with a certain distance therebetween (i.e., with a certain cell gap). A liquid crystal material fills the cell gap to complete the formation of the liquid crystal panel of the LCD.
The above conventional LCD reproduces a picture image in accordance with an electrical signal received from a video processor. FIG. 2a shows a schematic circuit diagram of the LCD of FIG. 1 and FIG. 2b shows a schematic circuit diagram of a pixel part of the LCD of FIG. 1. Using FIGS. 2a and 2b, the working principle of the conventional LCD will be explained hereinbelow.
Referring to FIGS. 2a and 2b, a signal voltage is selectively applied to the gate lines 13 arrayed in a vertical direction. The signal voltage is applied to the first one of the gate lines 13 at first, then to the second one of the gate lines 13, then to the third one of the gate lines 13, and so on. That is, the signal voltage is sequentially applied to the N gate lines. The signal voltage is a pulse type waveform having a voltage of 2-5 volts with the applying time of 60-65 .mu.sec. When the signal voltage is applied to a certain gate line 13, the gate electrodes 23 connected to that gate line 13 are applied with a scan voltage and the TFTs 17 connected to the same gate line 13 are turned on.
During the ON-state of the TFTs 17, electrical picture data from the corresponding source line 15 is transmitted from the source electrodes 25 to the drain electrodes 27 of the TFTs 17. Then the picture data is transmitted to the pixel electrodes 11 connected to the drain electrodes 27. Each signal of the picture data has two possible states, "0" and "1". The "0" state reflects no voltage difference between the corresponding pixel electrode and the common electrode, whereas the "1" state reflects a voltage difference between the corresponding pixel electrode and the common electrode. When there is a voltage difference, a voltage signal, e.g. 2-5 volts, is applied to the liquid crystal. When there is no voltage difference, a voltage signal, e.g. 0-1 volt, is applied to the liquid crystal. As a result, a voltage signal of 2-5 volts ("1" state) or 0-1 volt ("0" state) is applied to the pixel electrodes 11.
Once the scan voltage is applied to the gate line 13, before another scan voltage can be applied to the gate line 13, the gate line 13 is applied with a zero volt. The TFTs 17 connected to the gate line 13 are then switched to the OFF state. During that period, the stored capacitance at the pixel electrode is isolated and the pixel electrode remains in its present state until the next data signal is input as the next scan signal is applied.
An electrical field is applied to the liquid crystal in accordance with the voltage difference between the pixel electrode 11 and the common electrode 7. As the electrical field is formed, the arrangement direction of the liquid crystal molecules changes. Without the electrical field, the arrangement direction of the liquid crystal molecules is unchanged. Therefore, depending on whether or not the electric field is applied to the liquid crystal, a backlight from the backside of the panel is selectively transmitted through the liquid crystal layer, and a picture is reproduced on the screen of the LCD device. In FIG. 2b, the capacitance C.sub.LC of a liquid crystal capacitor 31 is the capacitance of a capacitor comprising the pixel electrode 11 and the common electrode 7.
In the conventional LCD having the above-described structure and operation, the gate line, the source line, the pixel electrode and the common electrode each are formed with a conductive material. Using an insulation material such as a silicon oxide or a silicon nitride, the conductive elements are electrically isolated. Then a time delay problem arises due to the resistance R of the conductive material and the capacitance C of the conductive material, as shown in equation (1). EQU T.sub.delay =R*C (1)
Particularly, a pulse signal applied to the common electrode 7 can be distorted by the time delay T.sub.delay arising due to a parasitic capacitance 33 (Csc) formed between the source line 15 and the common electrode 7 and the resistance of common electrode (Rc), as shown FIG. 2c.
When a scan voltage is applied to the gate line 13, a horizontal crosstalk problem occurs because of the distortion of the scan voltage driving the TFTs 17. The horizontal crosstalk causes failure in the reproduction of original picture data by the horizontally neighboring picture areas or the horizontally neighboring pixels. For example, assume that a picture is reproduced at the Xth row of a picture area 41 as shown in FIG. 3a. At that time, a scan voltage is applied to the Xth row for selecting and turning on the Xth row. The picture data applied to the data bus line is reproduced on each picture area 41 of the Xth row. However, with the actual picture values of 0, 0, 63, 0, 0 . . . shown in FIG. 3a, the represented picture values of 0, .alpha., 63, .alpha., 0 . . . are present in accordance with the distortion of the waveform of the common electrode 7 as shown in FIG. 3b. Therefore, in the conventional LCD, image quality deteriorates in the horizontal direction.
Reducing the parasitic capacitance (Csc) or the resistance of the common electrode (Rc) in equation (1) may correct the horizontal crosstalk problem. In the first case where the parasitic capacitance Csc is reduced to correct the horizontal crosstalks, parasitic capacitance Csc is determined using the dielectric constant of an insulation material covering the source line and the width of the source line. These values vary in accordance with the selection of an insulation material. By selecting an insulation material which reduces parasitic capacitance, it is possible to correct partially the horizontal crosstalk problem. However, there is a limitation in selecting a proper insulation material.
Instead, the present invention suggests a solution of reforming the structure of a color filter panel. To do so, the conventional method for manufacturing a conventional color filter panel is first investigated. FIG. 4 shows a plan view of a conventional color filter panel and FIGS. 5a-5e show cross-sectional views, taken along line V--V, explaining a conventional method of manufacturing the color filter panel of FIG. 4.
As shown in FIG. 5a, an opaque metal such as chromium is deposited on a transparent glass substrate 1a. The deposited opaque metal is patterned to form a black matrix 5 (FIG. 4) between the pixels in a grid pattern. The black matrix 5 is composed of a plurality of horizontal black matrix strips 5b and vertical black matrix strips 5a. The vertical and horizontal black matrix strips 5a and 5b are disposed at positions corresponding to the gate and source lines. The width of each of the black matrix strips 5a and 5b is greater than the width of the gate and source lines. The vertical black matrix strip 5a is formed corresponding to the position of the source line, and the horizontal black matrix strip 5b is formed corresponding to the position of the gate line.
As shown in FIG. 5b, a resin having a color among red, green and blue is selectively coated over the substrate 1a and the black matrix strips 5a and 5b. For example, a red resin is coated and patterned to form a plurality of red color filters 3R arranged with a predetermined space therebetween. Next, as shown in FIG. 5c, a green resin is coated and patterned to form a plurality of green color filters 3G arraying near the red color filters 3R. Finally, a blue resin is coated and patterned to form a plurality of blue color filters 3B arraying between the green color filters 3G and the red color filters 3R. In the step of forming such color filters, certain portions of the vertical black matrix strips 5a are exposed between the color filters 3R, 3G and 3B. As shown in FIG. 5d, however, the horizontal black matrix strips 5b formed along the gate lines are completely covered by the color filters.
As shown in FIG. 5e, indium tin oxide (ITO) is deposited on the color filters 3R, 3G and 3B to form a common electrode 7. In this case, the brightness rate (aperture ratio) is reduced and power consumption of the LCD increases, because the width of the vertical black matrix strips is greater than that of the corresponding source line. The wider the width of the vertical black matrix strip is the lower the aperture ratio becomes. Therefore, by reducing the width of the black matrix strips, an LCD panel with a high aperture ratio can be obtained. FIGS. 6 and 7 depict a general LCD panel having black matrix strips with reduced width.
In the color filter panel of the LCD shown in FIGS. 6 and 7, the width of the vertical black matrix strips 5a is smaller than that of the uncovered part of the source lines of the active panel. Because the width of the vertical black matrix strips 5a is small, the color filters 3R, 3G and 3B contact each other and completely cover the black matrix strips 5a and 5b. For example, as shown in FIG. 7, two of the color filters 3R, 3G and 3B completely cover the strips 5a and contact each other on the strips 5a. Further, the red color filter 3R completely covers the horizontal black matrix strip 5b.
Comparing the LCD panels of FIGS. 5e and 7, in the LCD panel shown in FIG. 5e, the vertical black matrix strips 5a formed between the color filters 3R, 3G and 3B are electrically connected to the common electrode 7. The resistance of the common electrode 7 (Rc) is then lower than the resistance of the ITO (R.sub.ITO), because the resistance Rc is connected with the resistance of the black matrix (R.sub.BM) in parallel. ITO is used to form the common electrode. The following equation (2) depicts the relation between different resistances. EQU Rc=R.sub.ITO *R.sub.BM /(R.sub.ITO +R.sub.BM)&lt;R.sub.ITO (2)
According to equation (2), although crosstalks are reduced in the color filter panel of FIG. 5e, other problems still exist.
On the other hand, in the LCD panel shown in FIG. 7, the common electrode 7 does not directly contact the vertical and horizontal black matrix strips 5a and 5b formed on the color filters 3R, 3G and 3B. Then the resistance of the common electrode (Rc) is the same as the resistance of the ITO (R.sub.ITO) as shown in the following equation (3). EQU Rc=R.sub.ITO (3)
Therefore, in the LCD panel of FIG. 7, a crosstalk problem still exists due to the resistance of the ITO, and the quality of the picture produced by this LCD device is worse than the quality produced by the LCD panel shown in FIG. 4.